The field of the invention relates to controlling the level of hydrocarbon emissions from an internal combustion engine. More specifically, the invention relates to controlling hydrocarbon emissions which may result by purging the fuel vapors from a fuel vapor recovery system into the internal combustion engine.
Motor vehicles of recent years are required to have a fuel vapor recovery system to reduce the amount of fuel vapors released into the atmosphere. Typically, a canister containing a fuel vapor absorbing material, such as activated charcoal, is coupled between the fuel system and the air/fuel intake of the engine. A purge valve positioned between the canister and air/fuel intake enables the periodic purging of fuel vapors from the canister dependent upon engine operating parameters. Systems of this type are disclosed in U.S. Pat. Nos. 4,308,842 issued to Watanabe et al, 4,326,489 issued to Heitert, 4,377,142 issued to Otsuka et al, and 4,411,241 issued to Ishida.
A problem with these recovery systems is that the purged fuel vapors inducted into the engine may alter the air/fuel ratio thereby increasing hydrocarbon emissions. An approach directed to this problem is diclosed in both U.S. Pat. No. 4,013,054 issued to Balsley et al and Japanese Pat. No. 57-86555 by Yanagisowa, wherein the purge flow rate is regulated in response to a feedback signal indicative of the oxygen level in the engine exhaust.
The inventors herein have recognized that limiting the purge flow rate in response to an exhaust feedback signal does not solve the problem of hydrocarbon emissions described hereinabove. More specifically, the propagation time from the engine air/fuel intake to the engine exhaust delays the required correction to the purge flow rate. Accordingly, when a vapor purge is first initiated, the increase in hydrocarbon emissions caused by induction of fuel vapors cannot be corrected for a predetermined time. This perturbation in hydrocarbon emissions is dramatically increased in vehicles employing three-way catalytic converters (CO, HC, NO.sub.x) which are designed to operate in a narrow range of air/fuel ratios referred to as stoichiometry. For example, a small decrease in air/fuel ratio from 14.7 to 14.6 may decrease the efficiency of the converter for removing hydrocarbons by approximately 20% (see SAE 82066, entitled "The Measurement and Improvement of the Transient A/F Characteristics of an Electronic Fuel Injection System", by D. R. Hamburg and D. Klick, 1982). This disadvantage is particularly troublesome when vapor purge is frequently cycled, such as while driving in urban areas.